The present invention relates generally to a hydraulic control system for use in an automatic transmission, and more particularly to a hydraulic control system which is well adapted for use in an automatic transmission with overdrive means and which may operate to insure a driving force sufficient for a vehicle when the vehicle engine is driven at a full throttle opening.
Automatic transmissions in common use in automotive vehicles automatically provide optimum gear ratio change or shifting in response to vehicle speed and a throttle opening and as a result a driver need only depress an accelerator pedal and a brake pedal in order to effect appropriate vehicle operation including gear shifting. It has been general practice to provide a torque converter with a mechanical transmission mechanism for establishing three or four forward speed conditions, thereby effectively attaining torque conversion at a low speed drive as well. In this respect, the gear change ratios of a mechanical transmission may be automatically selected commensurate with the running condition of a vehicle. Typical of the mechanical transmission mechanisms is a planetary gear type transmission having a hydraulic control device. The hydraulic control device includes a throttle pressure valve adapted to provide a pressure (a system hydraulic pressure) proportional to an intake throttle opening of the vehicle engine, a governor pressure valve adapted to generate a pressure (governor pressure) commensurate with a rotational speed of an engine output shaft thereby detecting the speed of a vehicle, a change-over valve for providing a hydraulic pressure to a working cylinder in a frictional engaging device in the planetary gear type transmission, and a pressure regulating control valve for providing a hydraulic pressure (line pressure) variable depending on intake throttle opening and vehicle speed.
In a case where the gear change ratio is automatically established in accordance with intake throttle opening and vehicle speed, then the highest speed is maintained even if the accelerator pedal is fully depressed to its maximum extent at the highest speed gear shift position and at a high vehicle speed. With a multiple speed transmission, the total gear reduction ratio at the highest speed gear shift position is necessarily small. As a result, a sufficient driving force for the vehicle cannot be achieved within a vehicle speed range in which the highest speed is maintained, even if the acclerator pedal is fully depressed to its maximum extent, when a passing acceleration is required, when a driving force is required for normal road operation under specific conditions (upward load), or when acceleration for other purposes is required. Accordingly, there results lack of acceleration or decleration on an upward slope, with an accompanying lowering in the performance of the vehicle. This tendency is particularly prevalent with automatic transmission having overdrive means.
It is an object of the present invention to provide a hydraulic control system for use in an automatic transmission which provides improved acceleration performance at full opening of the throttle valve of the vehicle engine.
It is another object of the present invention to provide a hydraulic control system for a vehicle transmission wherein when a driver depresses an accelerator pedal, the transmission will not shift to the highest speed gear shift position at throttle openings over a given value, or the transmission shifts to a lower-speed-side shift position from the highest speed shift position thereby achieving a sufficient driving force for operation of the vehicle.